25p to 50i artifcats?

[Statten Roeg]

I am planning to shoot a short film at 25p, probably on a Sony 900. If this is converted to 50i to play on a normal PAL television screen will there be any artifacts from the interlacing process? Thanks!

[David Mullen ASC]

I am planning to shoot a short film at 25p, probably on a Sony 900. If this is converted to 50i to play on a normal PAL television screen will there be any artifacts from the interlacing process? Thanks!

 

It will be the same as 25fps film transferred to PAL. It's just a display method that you can't avoid if you are showing it on an interlaced-scan monitor.

[Statten Roeg]

Thanks for the reply David. That is what I thought, but I had been reading about field leading and saw a warning that if you had recorded on an interlaced format that leads with the odd lines and then played out to an interlaced format that leads with the even lines there would be a strange effect, so I started to worry that the same would be true of video recorded in a progressive mode. But perhaps I am misunderstanding the whole process: does a camera record video as either progressive or interlaced or is this just a way of condensing information for display purposes...? Do you know a good book to read that summarises all of the different video format issues, both technical, such as bit rates and compression, and also aesthetic like motion and flicker? Thanks again for the reply.

[David Mullen ASC]

Cameras vary in terms of how the record progressive-scan capture or even if they truly capture progressive scan.

 

Technically, in progressive-scan, images are captured as whole frames and if shown progressive-scan, displayed as whole frames too.

 

So if converted to interlaced-scan, all that you'd notice is that every two fields displayed are identical images, just containing the alternate lines in each field of the same image.

 

Many progressive-scan cameras record the image to tape in some other format. Some always convert the capture to interlaced-scan for recording.

 

For example, and F900 records in the "PsF" (Progressive Segmented Frame) format, where each frame is stored on tape as two fields. So if you shoot 24P, it records it as 24PsF, which is basically the same as 48i, except that it's not a 48i capture and it get converted to 24P again upon playback (hence why Sony made up this name PsF because of they didn't want people to think it was actually an interlaced-scan camera).

 

Since 50i is a display format too, it's pretty simple to show 25PsF on a 50i monitor.

 

Many of the prosumer video cameras will store progressive-scan capture in 1080i, which is interlaced-scan. This is partly because the HDV tape format specs only support 720P or 1080i recording.

 

Think of it as a camera that can capture images at certain speeds and a tape deck that can only record images at another speed. So in the case of shooting 25P and storing it as 50i or 25PsF, you've got a camera sensor "running" at 25 fps but a tape recorder deck running at 50 Hz.

 

But some cameras that people think of as being progressive-scan are not, hence the confusion. Some are progressive-scan capture but can only record interlaced-scan, but some are interlaced-scan capture that can convert and output progressive-scan. Some even convert this conversion back to interlaced-scan again. Confusing.

[Statten Roeg]

Thanks again David. Does that mean that with old interlaced format video cameras, say Betacam SP recording PAL, each field making up a frame was not identical to the other only made of the alternate lines, but rather there was a 1/50th of a second time difference in the recording of each field and whatever might be able to happen in such a short time could be in one field of a frame and not the other! Also, do you know a good book about all these issues and the history of video formats (I'm quite interested in how to recreate the effect of old television cameras, like that strange crispness on recordings of The Beatles on Ed Sullivan or the blown out tone leaking on older television clips, I suppose this could even be recreated in grading suites nowadays, but it would be interesting to find out the original reasons) Thanks for replying to my elementary questions!

[David Mullen ASC]

Thanks again David. Does that mean that with old interlaced format video cameras, say Betacam SP recording PAL, each field making up a frame was not identical to the other only made of the alternate lines, but rather there was a 1/50th of a second time difference in the recording of each field and whatever might be able to happen in such a short time could be in one field of a frame and not the other!

 

Yes, that's why you get a sawtoothed edge to moving objects when you combine the two interlaced-scan fields into a single frame for transfer to film. The object has moved slightly between the first and second field capture. It's also way interlaced-scan motion looks so smooth (hyper-real) though, since the motion is sampled at twice the rate (50 times per second instead of 25 times per second.) Or conversely, why 24P or 25P looks so strobey.

[Statten Roeg]

Thank you very much for your time David, you have been very helpful.

[Thomas James]

Are people really buying interlaced standard definition television monitors anymore? Whenever I go to the store all I see are those progressive scan flat panel high definition televisions or the progressive scan projectors. If you are poor you have buy a small HDTV and sit closer to it. Old fashioned interlace picture tube televisions are too costly to produce because they use more materials than a flat panel HDTV.

[Brian Drysdale]

There are still a lot of older standard def CRT sets in the UK and apart from some HD channels everything is PAL. Quite a few sets in the shops are standard def rather than HD, although many people are buying HD capable.

 

BTW I've got an old Panasonic television which produces great pictures and is a lot better than most LCDs that I've seen.

[Thomas James]

The only way an old standard definition television is going to look better than an HDTV LCD is if you feed it with interlaced standard definition content. If the LCD television is feed with high definition content it will blow away the standard definition CRT. CRT's have excellent contrast but because they are interlaced its like looking through a screen door. LCD's because they are progressive have a rock solid picture.

[Statten Roeg]

The way I understand it is that barring 1 or 2 HD channels in the UK all the digital channels are broadcast at the digital version of PAL which is called something like 625i SDI at an aspect ratio of either 14:9 or 16:9 depending on how sure they are that their market has widescreen TVs. Perhaps they broadcast a progressive signal too and then the digital boxes choose the interlaced version if it knows the TV is an old CRT. What I don't understand is how the old analogue TV stations are still broadcast. Do they have to use different antennas to broadcast at the different wavelengths? And since most shows are ingested onto servers from DigiBetas and then converted to MPEG 2 files for broadcast digitally, how do they then convert that MPEG 2 into an old style PAL analogue signal... I just don't know!

[Brian Drysdale]

In the UK all the programmes on PAL are interlace and unless you've got digital cable or satellite everything else is transmitted analogue. A digital conversion process is going on, however, some regions won't be converting to digital until 2012.

 

The BBC HD in UK will use MPEG4 and the last I heard they were testing 1080i, but that could change when the final system is in place.

 

I don't think you can compare HD televisions to SD sets. Certainly, quite a few standard def LCD sets are unimpressive.

[Statten Roeg]

In the UK all the programmes on PAL are interlace and unless you've got digital cable or satellite everything else is transmitted analogue. A digital conversion process is going on, however, some regions won't be converting to digital until 2012.

 

There are plenty of digital channels in the UK being transmitted by antennas and received through normal aerials, all of the Freeview channels are broadcast digitally in this way, they are SD but digital. The switchover is the switching off of the analogue signal, the digital signal has been rolled out nationwide. The SD equivalent of PAL is 625i SDI or something like that.

[Brian Drysdale]

I didn't throw in the Freeview to avoid confusion because it's part of the signals being transmitted by the current analogue transmitters.

 

I wouldn't get PAL confused with with the digital signal chain. PAL is a SD format and in the UK is transmitted 625 lines with 576 lines visible and is interlace. The SDI is a Serial Digital Interface.

 

http://en.wikipedia.org/wiki/Serial_Digital_Interface.

 

In this world it's 576i

[Statten Roeg]

Thanks for the reply Brian, is that right that the Digital signals of Freeview are being broadcast by the same aerials, I have been trying to find out the exact system that is used to play out and transmit so called terrestrial television in the UK but have been piecing together tidbits and can't find a place with all of the info together. I would put a post in the digitalspy.co.uk forums but they won't accept my email account. All the old play out codgers on that site get really wound up if people call digital signals PAL and maintain that PAL is an analogue only format and that the digital equivalent should be called 625i SDI, I also thought it was weird as I thought of SDI as a type of cable, but as I can't post there I can't get anything out of them... This post has deviated extremely from its original intention, still quite interesting though.

[Keith Walters]

Are people really buying interlaced standard definition television monitors anymore? Whenever I go to the store all I see are those progressive scan flat panel high definition televisions or the progressive scan projectors. If you are poor you have buy a small HDTV and sit closer to it. Old fashioned interlace picture tube televisions are too costly to produce because they use more materials than a flat panel HDTV.

Actually, Interlaced scan CRT-based colour TVs are still being sold in huge numbers in poorer parts of the world, precisely because they're so cheap! I bought a 20" CRT colour TV just last week for the equivalent of about US$90, and it had two normal stereo AV inputs and a component input as well as the usual analog tuner. There's nothing wrong with it, well, nothing that hasn't been wrong with any other 20" TV bought in the last 30 years! The large screen market has gone over completely to LCD and plasma, but for smaller screen sizes, the Chinese manufacturers can still turn out CRT TVs dirt cheap. For a large part of the world, the price difference between CRT and flat panel is quite significant.

 

A couple of weeks before that I bought a brand new 19" CRT computer monitor for the equivalent of about $US55, and the picture on that is miles better than the 17" LCD monitor I use at work!

 

Most cheaper flat-panel TVs do not give all that good a picture. You'll pay far more for a flat panel display that has a picture quality comparable to a cheap CRT TV.

 

CRT TVs will eventually disappear from the market, but not just yet.

[Keith Walters]

Here's a link to just one US retail chain taken at random:

Wal-Mart

They have plenty of CRT TV sets for sale, both small screen and large. All of them have digital tuners, which is an FCC requirement now. Note that contrary to what was predicted at least 10 years ago, virtually none of them have 16x9 tubes, although most have 16 x 9 underscan capability. These are not just clearance stocks either, I know for a fact that the Chinese are still bringing out new model CRT sets, basically a sideline for their domestic production.

 

Wal-Mart have an extensive range of Plasma and LCD models as well, which is pretty much par for the course with the retailers I've looked at so far.

[Nigel Smith]

In the UK all the programmes on PAL are interlace and unless you've got digital cable or satellite everything else is transmitted analogue. A digital conversion process is going on, however, some regions won't be converting to digital until 2012.

 

The BBC HD in UK will use MPEG4 and the last I heard they were testing 1080i, but that could change when the final system is in place.

 

I don't think you can compare HD televisions to SD sets. Certainly, quite a few standard def LCD sets are unimpressive.

I have a Philips 35" CRT that gives beautiful pictures, and regularly shows up the poor quality and heavy compression that all digital broadcasting uses - it's like jpeg artifacts around moving objects and pixelation. Did you know that in the UK the digital channels are not only limited to around 5Mbps data rates, but the data rates are variable? So if the adverts start on channel 4, the picture quality dips on E4?

I watched the football the other night on BBC SD [i'm still getting over that experience!] and since it was available on BBC HD, I'm assuming that they were using progressive cameras. I say that because I experience bad motion artifacts on players, particularly on wide shots, where movement was strobey and almost slow-mo looking.

Edited November 23, 2007 by Nigel Smith